Case hardened steel and method of manufacturing the same

ABSTRACT

A case hardened steel having small oxygen, sulfur, and phosphorous contents and consisting essentially of, by weight, 0.10-0.30% carbon, not more than 0.50% silicon, not more than 1.50% manganese, not more than 0.012% phosphorous, not more than 0.009% sulfur, 0.02-0.04% aluminum, not more than 0.0010% oxygen, and 0.01-0.02% nitrogen, and a member or members selected from the group consisting of 0.20-1.50% chromium, 0.10-0.35% molybdenum, and 0.20-3.0% nickel, the remainder being iron together with impurities, and and a method of manufacturing the same. 
     The method includes performing oxidizing refinement of strictly selected raw materials in a smelting furnace, absorbing and removing a slag which is on the smelted steel poured from the smelting furnace into a container, performing reducing refinement under the presence of a highly basic slag having a basicity of not less than 3 and an inert atmosphere, performing a vacuum degassing, performing reducing refinement in a reducing atmosphere, and performing sealed casting.

Technical Field

The present invention relates to a high-quality case hardened steelhaving excellent fatigue strength, durability life, and workability foruse in machine structural components of vehicles, industrial machinery,and so on; and a method of producing said steel.

BACKGROUND ART

Machine structural components must satisfy various properties includingthose relating to fatigue strength, durability life, workability and thelike. In particular, fatigue strength is becoming increasingly importantwith trends toward heavier loads and higher speed together with therequirements of higher performance in industrial machinery and vehicles.Studies are being made to develop a steel having higher fatigue strengthdurability life, workability, and the like.

In order to improve fatigue strength, a conventional method was propsedwherein an alloying element such as Ni or Mo is added in an appropriateamount to increase the material strength. In some applications, anotherconventional method was proposed which uses a special smelting methodsuch as VAR or ESR so as to control the solid texture and to reduce theamount of non-metallic inclusions.

However, in the conventional method of simply adding the alloyingelement, a satisfactory long durability life cannot be obtained in someapplications. Again, the latter method is costly and not suitable tomass-production.

DISCLOSURE OF INVENTION

The present invention has been made in consideration of this and isbased on various studies made on the influence of various alloyingelements on the fatigue strength of a resultant case hardened steel.Based on such studies, it was found that the cleanliness of the steel isvery important, i.e., a mere trace of oxide and sulfide inclusionsconsiderably reduces the fatigue strength, and that other impuritiesimpair the fatigue strength.

Based on these findings, according to the present invention, an Ocontent is set to be 0.0010% or less which is the minimum O content thatcan be achieved with the current vacuum degassing refinement technique,an S conzent is set to be 0.009% or less which is considerably smallerthan that in conventional steel, and the amount of impurity element P isalso set to be 0.012% or less, so as to greatly reduce the amount ofnon-metallic inclusions in the steel, thereby obtaining an excellentfatigue strength.

Since the steel according to the present invention has a very smallamount of impurities, it has excellent cold workability.

In a method of manufacturing a highly pure, case hardened steel havinglow oxygen, sulfur, and phosphorus contents according to the presentinvention, good raw materials of steel are selected and, after oxidizingrefinement in an electric furnace, are poured into a ladle. The smeltedsteel is subjected to dephosphorization during pouring into the ladle orthereafter.

The oxide slag on the smelted steel is absorbed by a vacuum slagcleaner. A highly basic slag having a basicity of 3 or more (a reducingslag having an excellent desulfurization property such that FeO +MnO≦0.5% (by weight) and CaO/SiO₂ /Al₂ O₃ =0.3 to 0.4) is prepared byelectric heating. Reducing refinement is performed to reduce the amountof S to 0.009% by weight or less, the amount of 0 to 0.0020% by weightor less, and the low amount of P, while bath temperature is controlled,an inert gas is introduced through double porous bricks, under thepresence of the highly basic slag, and the smelted steel is stronglyagitated. Subsequently, vacuum degassing is performed by a circulatingvacuum degassing apparatus such that vigorous circulating is performedduring 2/3 of the total treatment time while weak circulating isperformed during 1/3 of the total treatment time, thereby furtherreducing the amounts of O, N, and H. Reducing refinement is thenperformed by weakly agitating the smelted steel in a reducing atmosphereat a pressure higher than normal pressure to allow minute inclusions tofloat and be removed. Finally, sealed casting is performed to greatlyreduce the O content to 0.0010% by weight or less, the S content to0.009% by weight or less, and the P content to 0.012% by weight or less,which are greatly smaller than in the conventional steels, to provide ahighly pure, case hardened steel having only a slight amount ofnon-metallic inclusions.

The steel according to the present invention will be described below.

A steel according to a first invention consists essentially of, byweight, 0.10-0.30% carbon, not more than 0.50% silicon, not more than1.50% manganese, not more than 0.012% phosphorus, not more than 0.009%sulfur, one or more members selected from the group consisting of0.20-1.50% chromium, 0.10-0.35% molybdenum, and 0.20-3.0% nickel, and0.020-0.040% aluminum, not more than 0.0010% oxygen, and 0.0100-0.0200%nitrogen, the remainder being iron together with impurities. A steelaccording to a second invention is obtained by further including one ortwo members selected from the group consisting of 0.03-0.10% by weightof vanadium and 0.03-0.10% by weight of niobium in the steel of thefirst invention,

further improving the fatigue strength of the steel of the firstinvention. The third invention concerns the method of manufacturing ahigh-quality case hardened steel according to the first invention,characterized by comprising absorbing a slag, which is on the smeltedsteel poured from a smelting furnace into a separate container, with avacuum slag cleaner, performing reducing refinement by stronglyagitating the smelted steel while adjusting a bath temperature byelectrode heating under the presence of a highly basic slag having abasicity of not less than 3 and in an inert atmosphere at a pressurehigher than normal pressure, performing a vacuum degassing by acirculating vacuum degassing apparatus by performing strong circulatingduring a 2/3 period of a treatment time and weak circulating during a1/3 period of the treatment time, and performing reducing refinement byweakly agitating the smelted steel in a reducing atmosphere at normalpressure.

The reasons for limiting the contents of the respective components ofthe steel according to the present invention will be described below.

Carbon is an important element which must be included to achieve a corehardness by carburizing hardening. In order to achieve hardness HRC of30 to 45 for imparting a required fatigue strength in a gear, a shaft,or the like, carbon must be contained in the amount of at least 0.10% ormore. However, when C is contained in an excessive amount, it degradesmachinability and impact resistance after carburizing. For this reason,the upper limit of C content in steel is set to be 0.30%. The C contentis preferably 0.25% or less.

Silicon is an element necessary to improve deoxidation property andhardenability. If Si is contained in an amount exceeding 0.50%, itdegrades workability such as machinability or causes an abnormalcarburizing layer after carburization. For this reason, the upper limitof Si content is 0.50%. The Si content is preferably 0.35% or less.

Manganese is an element necessary to improve deoxidation anddesulfurization properties and hardenability. If Mn is contained in anamount exceeding 1.50%, it degrades the workability of the resultantsteel. Therefore, the upper limit of the Mn content is 1.50%.

Chromium is an element which is effective in improving hardenability andstrength after hardening and tempering. When Cr is added in a carburizedsteel component, it improves the hardness and the effective carburizingdepth of the carburizing layer. In order to obtain these effects, Crcontent must be 0.20% or more. Therefore, the lower limit of the Crcontent is 0.20%.

When the Cr content exceeds 1.50%, however, the steel tends to beexcessively carburized when carburizing is performed, causing problems.Therefore, the upper limit of the Cr content is 1.50%.

Nickel is an element which is effective in improving toughness of asteel after hardening and tempering. In the present invention, Ni isadded in an amount of 0.20% or more depending on a requiredhardenability and strength. When the Ni content is excessive, however,retained austenite in the carburizing layer after carburization becomesexcessive, degrading the surface hardness. Also, since Ni is anexpensive element, the upper limit of the Ni content is set to be 3.00%in view of economy.

Molybdenum is an element which is effective in improving a hardenabilityand toughness after tempering. When Mo is added in a carburized steelcomponent, it improves the hardness and the effective carburizing depthof the carburized layer of the resultant steel. According to the presentinvention, Mo is contained in an appropriate amount in accordance withrequired hardenability, strength, and carburizing property. The lowerlimit of the Mo content for achieving an expected high strength is setto be 0.10%. If the Mo content is excessive, however, a carbide forms inthe carburizing layer, the amount of retained austenite is increased,causing unpreferable effects. Therefore, the upper limit of the Mocontent is set to be 0.35%.

Aluminum is an element which serve as a deoxidizing agent upon smelting,is combined with nitrogen to form AlN in the smelted steel, and preventscoarsening of grain during carburizing, thus controlling fine grains. Ifthe Al content is less than 0.020%, these effects cannot be obtained; ifthe Al content exceeds 0.040%, large amounts of alumina inclusions form,degrading the cleanliness or machinability of the steel. Therefore, theAl content is set to be 0.020 to 0.040%.

Nitrogen is an element which is combined with aluminum to form AlN andprevents coarsening of grain during carburizing. If all the Al containedin the steel is used to form AIN, the N content must be 0.0100% or more.Therefore, the lower limit cf the N content is set to be 0.0100%. Whenthe N content exceeds 0.0200%, toughness of the steel is impaired.Therefore, the upper limit of the N content is set to be 0.0200%.

Oxygen is an element which forms oxide inclusions that degrade thepitching resistance of a gear and the like and are harmful for theworkability such as a machinability. The upper limit of the O content isset to be 0.0010%.

Phosphorus is an element which easily forms segregation in the resultantsteel in a banded structure. When P segregates in the grain boundaries,the steel is embrittled. Therefore, the upper limit of the P content isset to be 0.012%.

Sulfur is an element which exists mainly in the form of a sulfide and iseffective in improving a machinability. When the S content is high,however, anisotropy occurs in the resultant steel, or the cleanliness ofthe steel is impaired, adversely influencing the fatigue strength.Therefore, the upper limit of the S content is set to be 0.009%.

Vanadium and niobium are elements which are effective in preventscoarsening of grain during carburizing by forming carbo-nitride in asimilar manner as AlN. It is necessary to contain V and/or Nb in thesteel in the amount of 0.03% or more, respectively, to obtain desiredeffects. However, even if these elements are contained in amountsexceeding 0.10%, they are bonded with C in the steel, thus degradinghardenability. Therefore, the upper limits for these elements are set tobe 0.10%.

Best Mode of Carrying Out the Invention

The characteristic features of the steel of the present invention willbe described by way of examples in comparison with those of comparativeand ccnventional steels. Note that the steel according to the presentinvention is obtained by smelting in accordance with the manufacturingmethod disclosed by the present invention.

Table 1 shows the chemical components of sample steel.

                                      TABLE 1                                     __________________________________________________________________________    Chemical Composition (wt %)                                                   C    Si Mn P  S  Ni Cr Mo Al O   N   Nb V                                     __________________________________________________________________________    A 0.15                                                                             0.23                                                                             0.82                                                                             0.010                                                                            0.002                                                                            0.04                                                                             1.15  0.028                                                                            0.0008                                                                            0.0133                                       B 0.21                                                                             0.26                                                                             0.84                                                                             0.009                                                                            0.003                                                                            0.05                                                                             1.13  0.032                                                                            0.0009                                                                            0.0145                                       C 0.16                                                                             0.25                                                                             0.81                                                                             0.011                                                                            0.002                                                                            0.04                                                                             1.12                                                                             0.16                                                                             0.035                                                                            0.0008                                                                            0.0150                                       D 0.22                                                                             0.27                                                                             0.83                                                                             0.010                                                                            0.003                                                                            0.05                                                                             1.10                                                                             0.17                                                                             0.029                                                                            0.0010                                                                            0.0143                                       E 0.17                                                                             0.23                                                                             0.61                                                                             0.009                                                                            0.002                                                                            1.74                                                                             0.61                                                                             0.21                                                                             0.030                                                                            0.0008                                                                            0.0138                                       F 0.21                                                                             0.30                                                                             0.59                                                                             0.009                                                                            0.003                                                                            1.72                                                                             0.65                                                                             0.22                                                                             0.033                                                                            0.0007                                                                            0.0155                                       G 0.18                                                                             0.25                                                                             0.81                                                                             0.011                                                                            0.005                                                                            0.04                                                                             1.12  0.035                                                                            0.0010                                                                            0.0153                                                                            0.07                                     H 0.17                                                                             0.31                                                                             0.83                                                                             0.010                                                                            0.006                                                                            0.05                                                                             1.15  0.035                                                                            0.0009                                                                            0.0145                                                                            0.05                                                                             0.08                                  J 0.20                                                                             0.33                                                                             0.85                                                                             0.011                                                                            0.004                                                                            0.04                                                                             1.13                                                                             0.15                                                                             0.037                                                                            0.0010                                                                            0.0148                                                                            0.06                                     K 0.21                                                                             0.28                                                                             0.86                                                                             0.012                                                                            0.005                                                                            0.04                                                                             1.13                                                                             0.16                                                                             0.034                                                                            0.0008                                                                            0.0155                                                                            0.07                                                                             0.07                                  L 0.21                                                                             0.26                                                                             0.82                                                                             0.014                                                                            0.011                                                                            0.06                                                                             1.17  0.030                                                                            0.0010                                                                            0.0137                                       M 0.18                                                                             0.30                                                                             0.83                                                                             0.011                                                                            0.008                                                                            0.04                                                                             1.11                                                                             0.17                                                                             0.035                                                                            0.0013                                                                            0.0141                                       N 0.17                                                                             0.27                                                                             0.81                                                                             0.018                                                                            0.021                                                                            0.05                                                                             1.10  0.025                                                                            0.0021                                                                            0.0090                                       P 0.18                                                                             0.32                                                                             0.78                                                                             0.017                                                                            0.023                                                                            0.05                                                                             1.12                                                                             0.16                                                                             0.027                                                                            0.0019                                                                            0.0088                                       Q 0.20                                                                             0.30                                                                             0.63                                                                             0.019                                                                            0.019                                                                            1.70                                                                             0.65                                                                             0.20                                                                             0.030                                                                            0.0024                                                                            0.0083                                       __________________________________________________________________________

In Table 1, steels A to K are steels of the present invention, steels Land M are comparative steels, and steels N to Q are conventional steels.

Table 2 shows the results of an experiment for determining rollingfatigue strength, surface hardness, internal hardness, and effectivecarburizing depth for the test pieces each having a diameter 60mm×length 10 mm obtained from the sample steels presented in Table 1when these test pieces were carburized under carburizing conditions of a0.90% of carbon potential and a carburizing temperature of 930°×5 hours,held to stand at 850° for 20 minutes, oil-quenched, and tempered at 160°for 90 minutes.

The rolling fatigue strength was measured by using a Mori-type rollingfatigue tester. The effective carburizing depth was examined in terms ofa distance between a surface and a point at which the hardness was morethan Hv 531.

                  TABLE 2                                                         ______________________________________                                                                           Effective                                  Rolling Fatigue                                                                              Surface  Internal   Carburized                                 Strength (× 10.sup.7)                                                                  Hardness Hardness   case Depth                                 (B.sub.10)                                                                             (B.sub.50)                                                                              (Hv)     (Hv)     (mm)                                     ______________________________________                                        A    4.32    9.70      801    288      1.10                                   B    4.88    10.1      782    345      1.22                                   C    10.5    24.6      791    356      1.18                                   D    5.8     11.3      772    405      1.28                                   E    8.7     10.6      753    356      1.20                                   F    7.6     20.5      747    395      1.21                                   G    5.6     12.3      793    310      1.16                                   H    4.10    9.8       785    315      1.15                                   J    6.85    18.4      776    389      1.22                                   K    5.22    10.8      769    400      1.25                                   L    2.58    5.63      780    337      1.18                                   M    2.12    2.77      759    329      1.16                                   N    0.95    1.23      769    301      1.15                                   P    1.06    1.97      778    375      1.23                                   Q    1.83    2.66      746    397      1.26                                   ______________________________________                                    

As can be seen from Table 2, regarding the rolling fatigue strengths ofthe conventional steels N to Q, their rated lives (B₁₀) are 0.95 to1.83×10⁷ and average lives (B₅₀) are 1.23 to 2.66×10⁷. In contrast this,since the O or S content or the like is minimized in the steels A to Kof the present invention, oxide or sulfide inclusions are decreasedtherein, thus providing greatly superior rolling life strengths over theconventional steels. Namely, the

rated lives (B₁₀) are 4.10 to 10.5×10⁷ and the average lives (B₅₀) are9.7 to 24.6×10⁷.

The steels L and M as comparative steels are slightly increased as tothe rated lives (B₁₀) of 2.12×10⁷ and 2.58×10⁷ and the average lives(B₅₀) of 2.77×10⁷ and 5.63×10⁷ compared with the conventional steels dueto the higher S and O contents than those in the steels of the presentinvention. However, the rated and average lives of the steels L and Mare lower than those of the present invention.

Table 3 shows the results of an experiment for determining the warmforging property for test pieces when the test pieces are cut from thesample steels shown in Table 1 in a directicn perpendicular to therolling direction, and ncrmalized by air-cooling after heating underconditions of 920°×1 hour. The warm forging property was examined interms of a reduction of area when test pieces for tensile test eachhaving a diameter of 6 mm were formed and subjected to the tensile testunder conditions of a tensile temperature of 700° C. and a strain rateε=10 s⁻¹.

                  TABLE 3                                                         ______________________________________                                        Reduction of       Reduction of     Reduction of                              Area (%)           Area (%)         Area (%)                                  ______________________________________                                        A   87          F      86        L    79                                      B   86          G      85        M    77                                      C   87          H      84        N    75                                      D   86          J      86        P    74                                      E   87          K      84        Q    87                                      ______________________________________                                    

As can be seen from Table 3, the steels N and P as the conventionalsteels containing Cr and Mo have reduction of area of 74 and 75%,respectively, and the steel L and M as the comparative steels havereduction of area of 79 and 77%, respectively. In contrast to this, allof the steels A to K according to the present invention have highreduction of area of 84% or more, thus providing an excellent warmforging property.

Table 4 shows the results of an experiment for determining austenitegrain sizes of the sample steels shown in Table 1 when the sample steelswere carburized under conditions of carburizing temperatures of 930°C.×6 hours, 950° C.×5 hours, and 970° C.×4 hours.

Regarding the rolling temperature, the steels N to Q as the conventionalsteels were rolled at 1,050° C., and the steels A to K according to thepresent invention and steels L and M as the comparative steels wererolled at 1,200° C.

                  TABLE 4                                                         ______________________________________                                        Grain Size                                                                    930° C. × 6 Hr                                                               950° C. × 5 Hr                                                                 970° C. × 4 Hr                         ______________________________________                                        A   8.8         8.2           3.7(21%), 8.8(79%)                              B   8.1         7.9           7.5                                             C   8.5         7.6           7.4                                             D   8.7         8.0           4.6(4%), 8.3(96%)                               E   8.8         7.7           2.8(6%), 8.7(94%)                               F   8.4         7.9           7.4                                             G   9.6         10.3          9.1                                             H   10.1        9.4           9.6                                             J   9.8         9.8           9.4                                             K   9.7         9.2           9.8                                             L   8.1         7.6           3.1(68%), 8.8(32%)                              M   8.3         7.7           3.4(71%), 8.7(29%)                              N   8.4         7.4           2.2(90%), 9.3(10%)                              P   8.9         3.1(30%), 8.6(70%)                                                                          1.5(85%), 10.2(15%)                             Q   8.3         4.3(25%), 7.8(75%)                                                                          1.3(95%), 10.4(5%)                              ______________________________________                                    

As can be seen from Table 4, the grain coarsening of the steels N to Qas the conventional steels and steels L and M as the comparative steelsis considerable by high-temperature carburizing at 950° C. and 970? C.In contrast to this, the grain coarsening of the steels A to K accordingto the present invention is slight even when the steels are subjected tocarburizing at high temperatures of 950° C. and 970° C. In this manner,the steels according to the present invention have an excellenthigh-temperature carburizing property.

FIG. 5 shows the results of an experiment for determining the fatiguestrength, internal hardness and effective carburized case depth of testpieces prepared from steels A to Q shown in Table 1. The test pieceswere prepared each to have a smoothed portion of 8 mm, and weresubjected to carburizing, quenching, and annealing in the same manner asthe test for determining the rolling fatigue strengths as shown in Table2, except for the carburizing conditions of 930° C.×3 hours.

The fatigue strength was tested using an Ono-type rotation bendingtester. Note that the effective carburizing depth was examined in termsof a distance from a surface to a point at which the hardness is morethan Hv 531.

                  TABLE 5                                                         ______________________________________                                                                 Effective                                            Durability  Internal     Carburized Case Depth                                Limit (× 10.sup.7)                                                                  Hardness (Hv)                                                                              (mm)                                                 ______________________________________                                        A   63.8        305          0.70                                             B   66.2        357          0.76                                             C   75.6        363          0.81                                             D   80.8        421          0.87                                             E   87.3        373          0.84                                             F   90.0        411          0.86                                             G   61.7        336          0.72                                             H   65.8        338          0.75                                             J   74.6        411          0.85                                             K   82.1        421          0.88                                             L   58.7        323          0.72                                             M   57.2        316          0.70                                             N   55.5        310          0.71                                             P   70.3        385          0.85                                             Q   78.3        414          0.88                                             ______________________________________                                    

As can be seen from Table 5, the steel conventional steel which containsonly Cr among Ni, Cr, and Mo has a durability limit of 55.5×10⁷ and thesteels L and M as the comparative steels have durability limits of57.2×10⁷ and 58.7×10⁷. In contrast to this, the steels A and B accordingto the present invention have durability limits of 63.8×10⁷ and66.2×10⁷, thus having a greatly improved fatigue strength thanconventional steels.

The steels C and D accordng to the present invention which contain Crand Mo have a superior durability limit to the steel P as theconventional steel, and the steels E and F according to the presentinvention which contain Ni, Cr, and Mo have a superior durability limitto the steel Q as the conventional steel. Therefore, the presentinvention can greatly improve the fatigue strength of Cr, Cr-Mo, andNi-Cr-Mo steels.

As described above, according to the present invention, the S and Ocontents or the like in the steel are minimized, the amounts of theoxide or sulfide inclusions in the steel are reduced, and thecleanliness of the steel is thus greatly improved. As a result, thefatigue strength, durability life, and warm forging property of thestructural steel are greatly increased. The present invention provides ahigh-quality case hardened steel suitable for vehicles, industrialmachinery, and the like, and a method of manufacturing the same, whichhas a high practical applicability.

We claim:
 1. A case hardened steel consisting essentially of, by weight,0.10-0.30% of carbon, not more than 0.50% of silicon, not more than1.50% of manganese, not more than 0.012% of phosphorus, not more than0.009% of sulfur, 0.020-0.040% of aluminum, not more than 0.0010% ofoxygen, 0.0100-0.0200% of nitrogen and a member of members selected fromthe group consisting of 0.20-1.50% of chromium, 0.10-0.35% of molybdenumand 0.20-3.0% of nickel, the remainder being iron and inevitableimpurities, said steel having a high reduction of area or more than 84%in warm forging and rated rolling fatigue strength B₁₀ of 4.1-10.5×10⁷and an average rolling fatigue strength B₅₀ of 9.7-24.6×10⁷ aftercarburizing followed by quenching and tempering.
 2. A case hardenedsteel consisting essentially of, by weight, 0.10-0.30% of carbon, notmore than 0.50% of silicon, ot more than 1.50% of manganese, not morethan 0.012% of phosphorus, not more than 0.009% of sulfur, 0.020-0.040%of aluminum, not more than 0.0010% of oxygen, 0.0100-0.0200% ofnitrogen, a member or members selected from the group consisting of0.20-1.50% of chromium, 0.10-0.35% of molybdenum and 0.20-3.0% of nickeland a member or members selected from the group consisting of 0.03-0.10%of vanadium and 0.03-0.10% of niobium, the remainder being iron andinevitable impurities, said steel having a high reduction of area ofmore than 84% in warm forging and rated rolling fatigue strength B₁₀ of4.1-10.5×10⁷ and an average rolling fatigue strength B₅₀ of 9.7-24.6×10⁷after carburizing followed by quenching and tempering.
 3. A methodpurifying a case hardened steel consisting essentially of carbon,silicon, manganese, phosphorus, sulfur, aluminum, oxygen, nitrogen andat least one member selected from the group consisting of chromium,molybdenum and nickel, the remainder being iron together with inevitableimpurities, comprising:(i) smelting the steel in a smelting furnace inorder to oxidatively scour the steel; (ii) pouring the smelted steelinto a separate container, performing dephosphorization of the smeltedsteel and absorbing and removing slag which contains oxide from thesteel which floats on top of the smelted steel with a vacuum slagcleaner; (iii) conducting reducing refinement of the steel by stronglyagitating the smelted steel in the presence of a highly basic slag whichhas basicity of not less than 3 while adjusting the temperature of thesteel bath, said refinement occurring under an inert atmosphere which isunder a pressure greater than normal pressure; (iv) performing vacuumdegassing of the steel with a circulating vacuum degassing apparatuswhich imparts strong circulation during two-thirds of the treatment, andweak circulation during one-third of the treatment; and (v) performing areducing refinement on the steel by weakly agitating the smelted steelin a reducing atmosphere at normal pressure, said treatment therebyreducing the amounts of phosphours, sulfur, oxygen, nitrogen andaluminum to not more than 0.012%, not more than 0.009%, not more than0.001%, 0.0100-0.0200% and 0.020-0.040% by weight, respectively.
 4. Themethod of claim 3, wherein said case hardened steel consists essentiallyof, by weight, 0.10-0.30% of carbon, not more than 0.50% of silicon, notmore than 1.50% of manganese, not more than 0.012% of phosphorus, notmore than 0.009% of sulfur, 0.020-0.040% of aluminum, not more th an0.0010% of oxygen, 0.0100-0.00200% of nitrogen and a member or membersselected from the group consisting of 0.20-1.50% of chromium, 0.10-0 35%of molybdenum and 0.20-3.0% of nickel, the remainder being iron togetherwith inevitable impurities.